1. A comparative quantitative study of the effects of dihydroergotamine (DHE) and noradrenaline (NA) on the precapillary and postcapillary resistance vessels, the precapillary ‘sphincters’, and the capacitance vessels was performed in a skeletal muscle and a skin region in healthy humans and in patients with orthostatic symptoms; further, the vascular effects of the drugs were analysed in muscle, skin, intestine and kidney in cats before and after sympathectomy.

2. The two drugs evoked a similar pattern of response in the cutaneous vascular bed, i.e. they both constricted resistance and capacitance vessels, increased the ratio of pre-/post-capillary resistance, but did not significantly influence precapillary sphincters. The reactions were similar in skeletal muscle for NA and also for DHE, with the important exception that the latter drug usually elicited a moderate dilator response in the muscle resistance vessels.

3. The average constrictor responses of the capacitance vessels were significantly larger for DHE than NA in skin and also in muscle despite the fact that DHE did not much affect the resistance vessels in muscle.

4. The effects of DHE on the intestinal and renal vascular circuits in the cat were comparatively small.

5. Since the constrictor effect of DHE seems confined mainly to the capacitance vessels, the drug may have beneficial effects in circulatory disorders characterized by impaired venomotor regulation.

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